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[209.85.220.41]) by mx.google.com with SMTPS id o67sor4508019ywe.200.2019.01.28.13.26.34 for (Google Transport Security); Mon, 28 Jan 2019 13:26:34 -0800 (PST) Received-SPF: pass (google.com: domain of hannes@cmpxchg.org designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; Authentication-Results: mx.google.com; dkim=pass header.i=@cmpxchg-org.20150623.gappssmtp.com header.s=20150623 header.b=CoGlOMn7; spf=pass (google.com: domain of hannes@cmpxchg.org designates 209.85.220.41 as permitted sender) smtp.mailfrom=hannes@cmpxchg.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=cmpxchg.org DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=doeprTD75a3wN3TW5uXx5DNXAT9rchfp4Kk5IuJTQxU=; b=CoGlOMn7uIwWJDcMJfm+Rj/Qm/gD747PAjoLMDuaAsDCqLhZQSn/JpFhVuDGuDVsjF ZCc8zkEIpRrNHtUDlnkc1025YEepn5whj5fHLPPzwg/nGaOXJ4pD5bxCR2wI5TuxUC48 BWSV8pZ8PCO4Myh5kKyI+tOm9WAx/oi7OMNMgHZUxJn7ycktWfXZZsol5OtiRFtjo1HM WCiToVrQxB7D49DqB2cfga9Imv1GfrUhMWjHVChx4ubG1zTMX4qOhNIsAbTi2dC5vikw xTOivor07e6gs17gDt22abifNO9qMz2FPPi8SCfL2ZFPvFyjHEY7VVO+xC22K209SvVq OA/g== X-Google-Smtp-Source: ALg8bN44vJXUgSafJIwnnLviacYE7rvHH/Jn6crYS91qDV9NBrrZWH6O6kmvBO1Sv1E4zJ1C7+B4zg== X-Received: by 2002:a81:3d3:: with SMTP id 202mr22391227ywd.18.1548710793942; Mon, 28 Jan 2019 13:26:33 -0800 (PST) Received: from localhost ([2620:10d:c091:200::5:42c8]) by smtp.gmail.com with ESMTPSA id e3sm14662632ywe.63.2019.01.28.13.26.32 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 28 Jan 2019 13:26:33 -0800 (PST) Date: Mon, 28 Jan 2019 16:26:32 -0500 From: Johannes Weiner To: Suren Baghdasaryan Cc: gregkh@linuxfoundation.org, tj@kernel.org, lizefan@huawei.com, axboe@kernel.dk, dennis@kernel.org, dennisszhou@gmail.com, mingo@redhat.com, peterz@infradead.org, akpm@linux-foundation.org, corbet@lwn.net, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor Message-ID: <20190128212632.GD1416@cmpxchg.org> References: <20190124211518.244221-1-surenb@google.com> <20190124211518.244221-6-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190124211518.244221-6-surenb@google.com> User-Agent: Mutt/1.11.2 (2019-01-07) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: One thought on the v3 delta that I missed earlier: On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote: > +/* > + * psi_update_work represents slowpath accounting part while psi_group_change > + * represents hotpath part. There are two potential races between them: > + * 1. Changes to group->polling when slowpath checks for new stall, then hotpath > + * records new stall and then slowpath resets group->polling flag. This leads > + * to the exit from the polling mode while monitored state is still changing. > + * 2. Slowpath overwriting an immediate update scheduled from the hotpath with > + * a regular update further in the future and missing the immediate update. > + * Both races are handled with a retry cycle in the slowpath: > + * > + * HOTPATH: | SLOWPATH: > + * | > + * A) times[cpu] += delta | E) delta = times[*] > + * B) start_poll = (delta[poll_mask] &&| if delta[poll_mask]: > + * cmpxchg(g->polling, 0, 1) == 0)| F) polling_until = now + grace_period > + * if start_poll: | if now > polling_until: > + * C) mod_delayed_work(1) | if g->polling: With the polling flag being atomic now, this "if g->polling" line isn't accurate anymore. Since this diagram is specifically about memory ordering, this should move the g->polling load up to where delta is read and then refer to unordered local variables down here.